Horizontally Redundant Systems

There are several ways to increase availability with parallel redundant servers that provide both load balancing and failover. The following figure illustrates two replicate servers providing an N+1 failover system. An N+1 system has an additional server to provide 100% capacity should one server fail.

Figure 5–3 N+1 Failover System With Two Servers

The computing power of each server in Horizontally Redundant Systems above is identical. One server alone handles the performance requirements. The other server provides 100% of the performance when called into service as a backup.

The advantage of an N+1 failover design is 100% performance during a failover situation. Disadvantages include increased hardware costs with no corresponding gain in overall performance (because one server is a standby for use in failover situations only).

The following figure illustrates a system that implements load balancing plus failover that distributes the performance between two servers.

Figure 5–4 Load Balancing Plus Failover Between Two Servers

In the system depicted in Horizontally Redundant Systems above, if one server fails, all services are available, although at a percentage of the full capacity. The remaining server provides 6 CPUs of computing power, which is 60% of the 10 CPU requirement.

An advantage of this design is the additional 2 CPU latent capacity when both servers are available.

The following figure illustrates a distribution between a number of servers for performance and load balancing.

Figure 5–5 Distribution of Load Between n Servers

Because there are five servers in the design depicted in Horizontally Redundant Systems, if one server fails the remaining servers provide a total of 8 CPUs of computing power, which is 80% of the 10 CPU performance requirement. If you add an additional server with a 2-CPU capacity to the design, you effectively have an N+1 design. If one server fails, 100% of the performance requirement is met by the remaining servers.

This design includes the following advantages:

• Added performance if a single server fails

• Availability even when more than one server is down

• Servers can be rotated out of service for maintenance and upgrades

• Multiple low-end servers typically cost less than a single high-end server

However, administration and maintenance costs can increase significantly with additional servers. You also have to consider costs for hosting the servers in a data center. At some point you run into diminishing returns by adding additional servers.